An RF ID tag with a battery power supply or of an active type, which is attached to a merchandise article or the like, or carried by a person, transmits an RF signal at a predetermined frequency that carries an ID and other information related to the article or the person, so that the RF signal is received and the information is read out by a reader device. The read-out information is further processed by a computer or the like, so that the distribution of the article or the action of the person is monitored and managed. The active-type RF ID tag with battery power supply has a larger communication range than a passive-type RF ID tag that receives power from a reader/writer device in a contactless manner, and hence is practical in use. However, the active-type RF ID tag transmits an RF signal in a fixed cycle, has a risk of being tracked by a third party, and hence has a problem in the security. To address this security problem, there has been developed an improved active-type RF ID tag that responds only to a tag ID request transmitted by the reader/writer device.
PCT International Publication WO 97/43740 published on Nov. 20, 1997 describes radio frequencies identification device which includes an integrated circuit including a receiver, a transmitter and a microprocessor. The receiver and transmitter together form an active transponder. The integrated circuit is preferably a monolithic single die integrated circuit including the receiver, the transmitter and the microprocessor. Because the device includes an active transponder, instead of a transponder which relies on magnetic coupling for power, the device has a much larger range.
Japanese Patent Application Publication JP 2000-113130-A published on Apr. 21, 2000 describes an IC tag detection system with low power consumption. This system includes a plurality of IC tags provided with different set times of day. Each IC tag includes a communication circuit, a control unit, a power source unit for supplying power from a battery to them, and time measuring means. Each IC tag performs transmission at each prescribed set time of day. This system also includes a detector for detecting the presence or absence of the IC tags based on the communication with them. The detector has a communication circuit, and determines the presence or absence of reception from them successively at the respective set times of day of the respective IC tags. Since the IC tag receives no inquiry from the detector, the IC tag can avoid useless reaction and battery consumption.
Japanese Patent Application Publication JP 2001-251210-A published on Sep. 14, 2001 (which corresponds to U.S. Pat. No. 6,922,402-B1) describes a method of locking a frequency in a transmitter at each of two nodes in a full duplex link, without using a separate reference oscillator in each node. The method provides locking of transmission frequencies of both nodes in a full duplex link at the same time by utilizing information of a received frequency to tune carrier frequencies of the transmitters. The offset of the carrier frequency of the fist transmitter is detected as the offset of a second corresponding receiver. The second receiver shifts the carrier frequency of the second transmitter, in response to the detected offset, to inform the first transmitter about the detected offset. The first receiver uses the detected offset to correct the carrier frequency of the first transmitter.
Japanese Patent Application Publication JP 2005-141529-A published on Jun. 2, 2005 describes an information communication system. In this system, an information processor includes a host and a reader/writer, and further includes an encryption/key storage unit and a data division/reconstitution unit for dividing data stored in a storage medium. The storage medium includes a plurality of RF ID devices which are independently driven. The information processor divides data stored in the storage medium, encrypts divided pieces of data with respective different encryption keys and stores these encrypted divided pieces of data in respective RF ID devices. This allows data stored in the storage devices to be confidential on a communication path.
U.S. Pat. No. 6,980,795-B1 issued on Dec. 27, 2005 (which corresponds to European Patent Application Publication EP 1104213-A2 and to Japanese Patent Application Publication JP 2001-189721-A) describes a wireless network with a cipher key change procedure. The wireless network employs a radio network controller and a plurality of assigned terminals which are provided for coding certain data to be transmitted over traffic and control channels and which are provided for changing the respective key necessary for the coding at certain instants. The radio network controller sends a message about a change of the key coded with an old cipher key to a terminal. The terminal responds with a message coded with a new cipher key as an acknowledgement for the new cipher key.